(a) Field of the Invention
The present invention relates to an apparatus for holding molten metal in continuous hot dip coating of a metal strip. More particularly, the present invention relates to a molten metal holding apparatus for the continuous hot dip coating of a metal strip, in which a metal strip is passed through a vessel filled with a molten coating metal and an electromagnetic field is used during the coating process to stably float the molten metal.
(b) Description of the Related Art
In continuous hot dip coating of metal strips, metal strips are continuously passed through a vessel filled with a molten metal, which is used as a coating solution. As shown in FIG. 15, in the conventional continuous hot dip coating method, a vessel 83 is filled with a molten metal 81, which is obtained by melting a metal by using as a metal solution aluminum, zinc, or an alloy of these metals, and a metal strip 89 that is continuously supplied to the vessel 83 using a sink roll 85 and a stabilizing roll 86 is dipped in the molten metal 81, after which the metal strip 89 is removed from the vessel 83.
The sink roll 85 acts to change a direction at which the metal strip 89 travels, and the stabilizing roll 86 acts to adjust the conveying state of the metal strip 89. The sink roll 85 and the stabilizing roll 86 are submerged in the molten metal 81 in the vessel 83, and axis members of the sink roll 85 and the stabilizing roll 86 are supported by a sleeve-bush configuration and without the use of lubrication as a result of the high temperature environment of inside the vessel 83.
At this time, parts forming the sink and stabilizing rolls 85 and 86 react with the molten metal 81 to generate metal compounds. If impurities created as a result adhere to a surface of the metal strip 89, the metal strip 89 is compressed in this state to reduce the quality of the metal strip 89.
Further, the rotation of the axis members of the sink and stabilizing rolls 85 and 86 without the use of lubricant results in wear of the axis members. This causes the metal strip 89 to vibrate to thereby result in defects such as a streaked pattern formed on the metal strip 89 or differences in the amount of coating.
To solve such problems, it is necessary to use a vessel structure in which such rolls are not submerged in the molten metal. In this regard, a molten metal process is disclosed that eliminates the use of metal strip support rolls that are submerged in the molten metal. In such a process, an opening through which the metal strip is supplied is formed in a lower section of a vessel. A metal strip to be plated is supplied to a lower portion of the molten metal through the opening then removed from the vessel through an upper section thereof. A configuration for preventing the molten metal from exiting through the opening is provided.
With regard to the configuration for preventing the molten metal from exiting through the opening in such a process where rolls submerged in molten metal are not used, Japanese Patent Laid-Open No. 63-109148 discloses a method in which gas pressure obtained by a gas pressure chamber mounted in the vicinity of the opening of the vessel is used to support the weight of the molten metal so that it floats. Also, Japanese Patent Laid-Open No. 63-303045 discloses a method in which a direct-current (DC) magnet is mounted in the area of the opening to supply a direct current to the molten metal such that it floats by the generated electromagnetic force.
In addition, U.S. Pat. No. 5,665,437 and Japanese Patent Laid-Open No. 63-310949 mount a linear induction motor in the area of the opening of the vessel to form a traveling magnetic field. The electromagnetic force formed as a result floats the molten metal. U.S. Pat. No. 5,897,683 discloses a holding method that uses an electromagnetic force generated by an alternating-current (AC) electromagnet mounted in the vicinity of the opening of the vessel and a conducting block in a specific area of the vessel, and uses a gas pressure obtained by providing a gas pressure chamber below the opening so that the molten metal does not exit the opening.
However, among the configurations and processes disclosed as described above, in the methods using gas pressure to float the molten metal, it is difficult to maintain a uniform pressure of the gas pressure chamber and a significant noise is generated. Also, if the gas permeates the molten metal, bubbles may form within the molten metal.
In the methods of holding the molten metal using a DC magnet and a DC source, DC current may pass through the metal strip to affect peripheral equipment. This poses safety risks to users.
Further, in the method of mounting a linear induction motor in the area of the vessel opening to float the molten metal, the metal strip passing through the opening may be deformed.
Finally, in the method of simultaneously using the AC electromagnet and the gas pressure chamber to float the molten metal, significant costs are involved by using both these configurations and gas may permeate the molten metal to form bubbles therein. Also, not only is it difficult to maintain the original shape of the conductor dipped in the molten metal, but also it is difficult to maintain the chemical composition of the molten metal itself.